As one example of an integrated circuit (IC) having a power supply circuit, consider a dynamic random access memory (DRAM) formed as an integrated circuit. Such an IC conventionally accepts an externally applied power signal (V.sub.CCX) on one of its contacts. To operate, V.sub.CCX is applied with a voltage in a range including 5 volts measured relative to a ground contact. For retaining data stored in the memory while it operates, V.sub.CCX is supplied from a battery in a range including 3.3 volts. The reliability of such a DRAM design is commercially important. Therefore, individual DRAM ICs are tested at elevated temperatures and at elevated values of V.sub.CCX, such as a voltage about 8 volts, to identify devices that do not conform to design specifications. These tests are called burn-in tests. Design specifications, some of which are measured during burn-in tests, are published and guaranteed by the manufacturers of DRAMs.
The circuits that make up the conventional 3.3 volt DRAM include a power supply circuit that receives V.sub.CCX and provides an optimal value for an internal voltage V.sub.CCR. The value of V.sub.CCR cannot be obtained by measurement after the DRAM has been encapsulated. Process variation from lot to lot and from device to device in an extreme case can produce a percentage of DRAMs each having a power supply circuit wherein the transition points for the piecewise linear function of V.sub.CCR as a function of V.sub.CCX vary widely from design values. This variation limits the effectiveness of DRAM testing, especially burn-in screening for the removal of weak parts from inventory to be sold.
In addition to the lack of burn-in screening effectiveness, testing for purposes of guaranteeing operating specifications is also made difficult by conventional DRAM power supply circuitry. The conventional power supply circuit provides a regulated value for V.sub.CCR over a wide range of values of V.sup.CCX. Since many operating specifications are affected by V.sub.CCR, the fact that V.sub.CCR is not measurable in an encapsulated DRAM makes selection of a guaranteed specification a difficult empirical exercise. To avoid a large number of devices found to not conform to the guaranteed specification after delivery, manufacturers pad the specification to allow for process variation, testing tolerances, and temperature variation, to name a few significant variables. If on the other hand, the padded specification is too conservative, many superior devices will be sold as if they were lower grade devices and the premium price for superior devices cannot be collected.
These problems are not unique to DRAM manufacturing. Identical problems assail the manufacture of any integrated circuit type, including logic circuits, microcontrollers, microprocessors, memory devices, analog and digital converters, analog circuits, amplifiers, receivers, modulators, video circuits, and digital signal processors, to name a few representative types.
In view of the problems described above and related problems that consequently become apparent to those skilled in the applicable arts, the need remains in the manufacture of integrated circuits for an improved power supply circuit.